There was no official confirmation from the Turkish military, but a senior Turkish security source told Reuters news agency on Wednesday that the observation posts were being reinforced.

"We have a military presence there and if that military presence is damaged or attacked in any way, it would be considered an attack on Turkey and would therefore receive the necessary retaliation," the source said.

Why are there observation posts in Idlib?

Turkey established the observation posts to monitor a "de-escalation" agreement with Russia and Iran, which was intended to reduce violence along the frontlines.

The agreement also covered three other conflict zones in Syria that were recaptured earlier this year by the Syrian army with the support of Russian air strikes and Iranian-backed militias.

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Syrian rebels have also been reinforcing their positions in Idlib, Hama and Aleppo

The Syrian government has said it now intends to "liberate" Idlib from Hayat Tahrir al-Sham (HTS), an al-Qaeda-linked jihadist alliance whose estimated 10,000 fighters control large parts of the province.

Foreign Minister Walid Muallem has said it hopes to avoid civilian casualties and regain territory through "reconciliation agreements", but that it is determined to defeat HTS "no matter the sacrifices".

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The region is home to 1.4 million people displaced from other parts of Syria

He also warned that Turkey, which is already hosting 3.5 million Syrian refugees, would not be able to absorb the 800,000 people whom the UN believes would be displaced by an all-out assault on Idlib.

A senior UN aid official, Panos Moumtzis, on Thursday quoted a Russian official as telling a humanitarian taskforce in Geneva that "every effort to find a peaceful solution to the problem is being made".